Effect of annealing temperature on structural, optical and magnetic properties of green synthesized ZFO nanoparticles for electrochemical energy storage applications

Abstract

The aim of this work is to explore the effect of annealing temperature on structural, optical, magnetic properties and electrochemical properties of green synthesized ZnFe2O4 (ZFO) nanoparticles (NPs) for energy storage applications. ZFO NPs annealed at 400 °C sample showed an enormous improvement in electrochemical energy as compared to as-synthesized and other NPs annealed at different temperatures because of its higher surface area and mesoporous distribution of pore sizes. Reitveld refinement analysis of HR-XRD spectra revealed the formation of cubic spinel phase with Fd-3 m (227) space group and crystallite size significantly increased when annealing increased from 400 °C to 1000 °C because of Oswald ripening effect. The HR-TEM showed a clear irregular cubical structure of ZFO NPs with an average size of 61.20 nm for 1000 °C annealed sample. Fourier transform infra-red spectra of ZFO NPs shows metal-oxide peaks of ZnO and FeO at wavenumbers 525 cm−1 and 353 cm−1 respectively. The optical bandgap of ZFO NPs decreases with increasing temperature due to increase in the crystallite size. In addition, it is observed that, ZFO NPs showed superparamagnetic nature at room temperature and ferromagnetic nature at low temperature. The ZFO NPs annealed at 400 °C exhibit the highest specific capacitance of 435.45 F/g at 1 A/g current density. Thus, annealing temperature significantly effects the properties of green synthesized ZFO NPs. We believe the cost effective green synthesized spinel structure of ZFO NPs has great potential for electrochemical energy storage applications.